In recently published articles and patents there were described a number of methods for measuring bulk fluid compositions in statis and flow by visible light reflection characteristics from the optically inhomogeneous wall of a fluid containing vessel. Surface Plasmon Spectroscopy (SPS) on gratings and randomly rough surfaces has proven to be especially sensitive to fluid composition near the wall. See M. J. Jory, P. S. Vukusic, J. R. Sambles, Sensors and Actuators B: Chemical 17, 203-209 (1994). It has been shown that SPS combined with electrochemical modulation techniques allow for the determination of relative concentrations of surface active species as low as 10.sup.-15 to 10.sup.-14 M. See A. M. Brodsky, M. Urbach, Sov. Phys. Uspechi, 132, 413 (1982).
SPS techniques in general have a number of inherent drawbacks when used as analytical sensing techniques. For instance, the metal dielectric interface that is necessary for surface plasmon generation must be chosen carefully. As only a limited number of metals (gold, silver) are available for the SPS, the technique is limited to environments that will not corrode or destroy the surface plasmon generation medium. In contrast, the technique described herein, termed Grating Light Reflection Spectroscopy (GLRS), relies on an optical phenomenon that requires only a sufficient dielectric difference between the substrate and "sensing" layers. Thus, any combination of substrate and grating materials with sufficient dielectric difference will suffice, allowing for sensing in environments that would preclude the use of SPS.
Additionally, the surface sensitivity associated with SPS techniques make them unstable for real world analytical applications as the bulk determinations of solutions would be unattainable in the presence of surface active analytes. The techniques described herein proves to be relatively insensitive to surface active species. Although, here, we have not shown this effect experimentally, the mathematics described below indicate a relative surface insensitivity.